For many years bubble balancers have been widely used in automotive service shops for use in statically balancing the wheels of automotive vehicles. Such balancers generally include a pivot head to which the wheel to be balanced is mounted. The pivot head pivotally supports the wheel about a pivot point or area located on the axis of rotation of the wheel above its center of gravity. A 360.degree. spirit level is mounted to the pivot head in parallel relationship to the principal plane of the wheel so that the bubble in the level is centered only when the center of gravity of the wheel lies on the axis of rotation thereof, i.e., directly below the pivot point. When this condition occurs, the principal plane of the wheel is, of course, horizontal.
The prior art bubble balancers have generally utilized a pivot assembly comprising a substantially non-compressible ball and a substantially non-deformable platen. In some designs the platen is fixedly mounted to the top of an upright spindle or other support member and the wheel to be balanced is mounted to the ball which in turn rests on the platen. In other designs the ball is mounted to the top of an upright spindle or other support and the wheel to be balanced is mounted to the platen which in turn rests on the ball. It may thus be seen that with these two most commonly used designs the pivot point is located either at the area of contact between the platen and the top of the ball or between the platen and the bottom of the ball.
An inherent problem with the prior art bubble balancers has been the fact that they do not provide consistently repeatable indications of balance. Theoretically, if the balancer indicates that the wheel mounted thereon is balanced, removal of the wheel from the balancer and the subsequent replacement of the wheel thereon should cause the bubble in the level to return to the central position. All too frequently this is not the case, and neither the cause of the problem nor its solution has heretofore been found.
We believe that there are two principle reasons for the fact that the prior art bubble balancers have not always enabled repeatable balancing operations, and the reasons differ with the two types of pivot assemblies described above.
In those balancers wherein the ball rests on the platen, we have found that a true indication of balance will occur only when the platen surface is perfectly horizontal. Otherwise, the ball tends to roll downhill on the platen and thereby exert a torque on the pivot head causing it to come to rest in a non-horizontal position when the wheel under test is actually in balance. With this condition, rotation of a balanced wheel on the balancer will cause the bubble to move off center. Because of the portable nature of most bubble balancers and the conditions under which they are used, even though adjustment means are commonly provided for initially leveling the balancers, the platens are often times not horizontal during a balancing operation.
In those prior art balancers wherein the platen rests on the ball, the platen surface has been made concave to hold the platen on the ball. Theoretically, if the wheel is mounted with its axis of rotation centered in the recess such a concave surface should assure that the contact area between the ball and the platen lies on the axis of rotation of the wheel being balanced. However, when the wheel being balanced swings into a tilted condition, i.e., the principal plane thereof is not horizontal, the platen moves laterally and down across the surface of the ball thus causing the pivot area to move from the center position in the recess and thus be displaced from the axis of rotation of the wheel. Consequently, as the wheel oscillates or swings back and forth and slowly approaches a stationary horizontal position of balance, the platen does not always recenter itself on the ball inasmuch as there is but a very small component of force tending to cause the platen to move laterally to the critical center position. Yet, such balancers will only function correctly if the pivot area is on the axis of rotation of the wheel. Of course, if the platen has a recess whose radius is the same as that of the ball the platen cannot shift back and forth across the surface of the ball, but the friction between the platen and the ball reduces the accuracy of the balancer below acceptable limits. Another seemingly valid solution to this problem would be to reduce the size of the ball, but crushing or flattening of the ball or platen then becomes a problem of greater concern than non-repeatability.